This invention relates to an improved power switch circuit for turning on manually and for turning off either manually or automatically a battery operated electronic instrument.
A conventional power switch utilizes one "on" switch and one "off" switch for manually turning on or off the power to an electronic instrument. Generally, depressing the "on" switch provides a current to the base of a first transistor thereby turning it on. The first transistor conducts a signal to a timer circuit which produces a signal for turning on a second transistor that provides battery supplied power to the electronic instrument. The "on" switch is then bypassed and the power to the electronic instrument remains on until the timer, automatically after a fixed period of time elapses, turns off the second transistor or when the "off" switch is activated.
U.S. Pat. No. 4,112,764 issued to Turner discloses an automatic on/off digitally timed electronic switch which is responsive to momentary operation of an actuator to turn on and remain on for a predetermined interval of time. The switch has a latching circuit adapted for connection to a power supply and a drive circuit responsive to the momentary supply of power by operation of the actuator to provisionally begin operating and to provide a gating signal to operate the latching circuit. A means is provided for interconnecting the latching circuit and drive circuit for subsequently continuing the operation of the drive circuit independent of the momentary supply of power. A digital timer circuit includes a counter that is responsive to the momentary operation of the actuator to begin counting and to provide, after a predetermined count, an output signal to automatically inhibit further operation of the drive circuit and de-energize the latching circuit.
U.S. Pat. No. 3,965,743 also issued to Turner discloses an automatic electronic switch that is responsive to an actuator to turn on and provide power from a power supply to a circuit during a predetermined interval of time and to automatically turn off at the end of the predetermined time interval thereby turning off power to the circuit.
U.S. Pat. No. 3,621,276 issued to Mitchell discloses a circuit in which a supply battery is connected across a capacitor such that the capacitor charges to the voltage level of the supply battery to effect closure of a current path through the circuit. When the battery is disconnected across the capacitor, the capacitor discharges, thereby breaking the current path through the circuit after an interval of time determined by the rate of fall of the voltage across the capacitor.
It is therefore an object of this invention to provide an improved power switch circuit that is manually turned on and either manually or automatically turned off.
Another object of this invention is to provide an improved power switch circuit having only one momentary contact on/off switch for manually turning on or off the power supplied to an electronic instrument electrically connected to the power switch circuit.
A further object of this invention is to provide an improved power switch circuit having an electronic switch for reconfiguring the circuit when power is being supplied to an electronic instrument so that the on/off switch may again be manually activated to turn off the power.